[libcxx-commits] [libcxx] [libc++][ranges] Applied `[[nodiscard]]` to `iota_view` (PR #173612)
Hristo Hristov via libcxx-commits
libcxx-commits at lists.llvm.org
Fri Dec 26 00:06:45 PST 2025
https://github.com/H-G-Hristov created https://github.com/llvm/llvm-project/pull/173612
`[[nodiscard]]` should be applied to functions where discarding the return value is most likely a correctness issue.
- https://libcxx.llvm.org/CodingGuidelines.html
- https://wg21.link/range.iota
Towards #172124
>From b01c1db0f0ac4aecc04379f24f4fccf9fc69a8e0 Mon Sep 17 00:00:00 2001
From: Hristo Hristov <hghristov.rmm at gmail.com>
Date: Fri, 26 Dec 2025 10:06:25 +0200
Subject: [PATCH] [libc++][ranges] Applied `[[nodiscard]]` to `iota_view`
`[[nodiscard]]` should be applied to functions where discarding the return value is most likely a correctness issue.
- https://libcxx.llvm.org/CodingGuidelines.html
- https://wg21.link/range.iota
Towards #172124
---
libcxx/include/__ranges/iota_view.h | 34 +++++----
.../range.iota.view/nodiscard.verify.cpp | 75 +++++++++++++++++++
2 files changed, 93 insertions(+), 16 deletions(-)
create mode 100644 libcxx/test/libcxx/ranges/range.factories/range.iota.view/nodiscard.verify.cpp
diff --git a/libcxx/include/__ranges/iota_view.h b/libcxx/include/__ranges/iota_view.h
index f66f3f9183fc7..0ce157b8679e3 100644
--- a/libcxx/include/__ranges/iota_view.h
+++ b/libcxx/include/__ranges/iota_view.h
@@ -132,7 +132,8 @@ class iota_view : public view_interface<iota_view<_Start, _BoundSentinel>> {
_LIBCPP_HIDE_FROM_ABI constexpr explicit __iterator(_Start __value) : __value_(std::move(__value)) {}
- _LIBCPP_HIDE_FROM_ABI constexpr _Start operator*() const noexcept(is_nothrow_copy_constructible_v<_Start>) {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Start operator*() const
+ noexcept(is_nothrow_copy_constructible_v<_Start>) {
return __value_;
}
@@ -196,7 +197,7 @@ class iota_view : public view_interface<iota_view<_Start, _BoundSentinel>> {
return *this;
}
- _LIBCPP_HIDE_FROM_ABI constexpr _Start operator[](difference_type __n) const
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Start operator[](difference_type __n) const
requires __advanceable<_Start>
{
return _Start(__value_ + __n);
@@ -238,27 +239,27 @@ class iota_view : public view_interface<iota_view<_Start, _BoundSentinel>> {
return __x.__value_ <=> __y.__value_;
}
- _LIBCPP_HIDE_FROM_ABI friend constexpr __iterator operator+(__iterator __i, difference_type __n)
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI friend constexpr __iterator operator+(__iterator __i, difference_type __n)
requires __advanceable<_Start>
{
__i += __n;
return __i;
}
- _LIBCPP_HIDE_FROM_ABI friend constexpr __iterator operator+(difference_type __n, __iterator __i)
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI friend constexpr __iterator operator+(difference_type __n, __iterator __i)
requires __advanceable<_Start>
{
return __i + __n;
}
- _LIBCPP_HIDE_FROM_ABI friend constexpr __iterator operator-(__iterator __i, difference_type __n)
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI friend constexpr __iterator operator-(__iterator __i, difference_type __n)
requires __advanceable<_Start>
{
__i -= __n;
return __i;
}
- _LIBCPP_HIDE_FROM_ABI friend constexpr difference_type operator-(const __iterator& __x, const __iterator& __y)
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI friend constexpr difference_type operator-(const __iterator& __x, const __iterator& __y)
requires __advanceable<_Start>
{
if constexpr (__integer_like<_Start>) {
@@ -289,14 +290,14 @@ class iota_view : public view_interface<iota_view<_Start, _BoundSentinel>> {
return __x.__value_ == __y.__bound_sentinel_;
}
- _LIBCPP_HIDE_FROM_ABI friend constexpr iter_difference_t<_Start>
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI friend constexpr iter_difference_t<_Start>
operator-(const __iterator& __x, const __sentinel& __y)
requires sized_sentinel_for<_BoundSentinel, _Start>
{
return __x.__value_ - __y.__bound_sentinel_;
}
- _LIBCPP_HIDE_FROM_ABI friend constexpr iter_difference_t<_Start>
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI friend constexpr iter_difference_t<_Start>
operator-(const __sentinel& __x, const __iterator& __y)
requires sized_sentinel_for<_BoundSentinel, _Start>
{
@@ -336,24 +337,24 @@ class iota_view : public view_interface<iota_view<_Start, _BoundSentinel>> {
requires(!same_as<_Start, _BoundSentinel> && !same_as<_BoundSentinel, unreachable_sentinel_t>)
: iota_view(std::move(__first.__value_), std::move(__last.__bound_sentinel_)) {}
- _LIBCPP_HIDE_FROM_ABI constexpr __iterator begin() const { return __iterator{__value_}; }
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr __iterator begin() const { return __iterator{__value_}; }
- _LIBCPP_HIDE_FROM_ABI constexpr auto end() const {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto end() const {
if constexpr (same_as<_BoundSentinel, unreachable_sentinel_t>)
return unreachable_sentinel;
else
return __sentinel{__bound_sentinel_};
}
- _LIBCPP_HIDE_FROM_ABI constexpr __iterator end() const
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr __iterator end() const
requires same_as<_Start, _BoundSentinel>
{
return __iterator{__bound_sentinel_};
}
- _LIBCPP_HIDE_FROM_ABI constexpr bool empty() const { return __value_ == __bound_sentinel_; }
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr bool empty() const { return __value_ == __bound_sentinel_; }
- _LIBCPP_HIDE_FROM_ABI constexpr auto size() const
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto size() const
requires(same_as<_Start, _BoundSentinel> && __advanceable<_Start>) ||
(integral<_Start> && integral<_BoundSentinel>) || sized_sentinel_for<_BoundSentinel, _Start>
{
@@ -382,13 +383,14 @@ namespace __iota {
struct __fn {
template <class _Start>
requires(requires(_Start __s) { ranges::iota_view<decay_t<_Start>>(std::forward<_Start>(__s)); })
- _LIBCPP_HIDE_FROM_ABI constexpr auto operator()(_Start&& __start) const
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto operator()(_Start&& __start) const
noexcept(noexcept(ranges::iota_view<decay_t<_Start>>(std::forward<_Start>(__start)))) {
return ranges::iota_view<decay_t<_Start>>(std::forward<_Start>(__start));
}
template <class _Start, class _BoundSentinel>
- _LIBCPP_HIDE_FROM_ABI constexpr auto operator()(_Start&& __start, _BoundSentinel&& __bound_sentinel) const noexcept(
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto
+ operator()(_Start&& __start, _BoundSentinel&& __bound_sentinel) const noexcept(
noexcept(ranges::iota_view(std::forward<_Start>(__start), std::forward<_BoundSentinel>(__bound_sentinel))))
-> decltype(ranges::iota_view(std::forward<_Start>(__start), std::forward<_BoundSentinel>(__bound_sentinel))) {
return ranges::iota_view(std::forward<_Start>(__start), std::forward<_BoundSentinel>(__bound_sentinel));
@@ -402,7 +404,7 @@ inline constexpr auto iota = __iota::__fn{};
# if _LIBCPP_STD_VER >= 26
-inline constexpr auto indices = [](__integer_like auto __size) static {
+inline constexpr auto indices = [] [[nodiscard]] (__integer_like auto __size) static {
return ranges::views::iota(decltype(__size){}, __size);
};
diff --git a/libcxx/test/libcxx/ranges/range.factories/range.iota.view/nodiscard.verify.cpp b/libcxx/test/libcxx/ranges/range.factories/range.iota.view/nodiscard.verify.cpp
new file mode 100644
index 0000000000000..80a20a90bae37
--- /dev/null
+++ b/libcxx/test/libcxx/ranges/range.factories/range.iota.view/nodiscard.verify.cpp
@@ -0,0 +1,75 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: std-at-least-c++20
+
+// Check that functions are marked [[nodiscard]]
+
+#include <ranges>
+#include <vector>
+// #include <utility>
+
+#include "test_macros.h"
+
+void test() {
+ // [range.iota.view]
+
+ auto v = std::views::iota(49, 94);
+
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ v.begin();
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ v.end();
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ v.end();
+
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ v.empty();
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ v.size();
+
+ // [range.iota.iterator]
+
+ auto it = v.begin();
+
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ *it;
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ it[82];
+
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ it + 1;
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ 1 + it;
+
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ it - 1;
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ it - it;
+
+ // [range.iota.sentinel]
+
+ auto st = v.end();
+
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ st - it;
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ it - st;
+
+ // [range.iota.overview]
+
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::views::iota(1);
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::views::iota(1, 10);
+
+#if _LIBCPP_STD_VER >= 26
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::views::indices(5);
+#endif
+}
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